Selective micro device transfer to receiver substrate

ABSTRACT

A method of selectively transferring micro devices from a donor substrate to contact pads on a receiver substrate. Micro devices being attached to a donor substrate with a donor force. The donor substrate and receiver substrate are aligned and brought together so that selected micro devices meet corresponding contact pads. A receiver force is generated to hold selected micro devices to the contact pads on the receiver substrate. The donor force is weakened and the substrates are moved apart leaving selected micro devices on the receiver substrate. Several methods of generating the receiver force are disclosed, including adhesive, mechanical and electrostatic techniques.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to Canadian Application No. 2,879,465,filed Jan. 23, 2015, Canadian Application No. 2,879,627, filed Jan. 23,2015, Canadian Application No. 2,880,718, filed Jan. 28, 2015, CanadianApplication No. 2,883,914, filed Mar. 4, 2015 and Canadian ApplicationNo. 2,890,398, filed May 4, 2015, each of which is hereby incorporatedby reference herein in its entirety.

FIELD OF THE INVENTION

The present disclosure relates to device integration into systemsubstrates. More specifically, the present disclosure relates toselective transfer of micro devices from a donor substrate to a receiversubstrate.

BRIEF SUMMARY

According to one aspect there is provided, a method of transferringselected micro devices in an array of micro devices each of which isbonded to a donor substrate with a donor force to contact pads in anarray on a receiver substrate, the method comprising: aligning the donorsubstrate and the receiver substrate so that each of the selected microdevices is in line with a contact pad on the receiver substrate; movingthe donor substrate and the receiver substrate together until each ofthe selected micro devices is in contact or proximity with a respectivecontact pad on the receiver substrate; generating a receiver force thatacts to hold the selected micro devices to their contact pads while notaffecting other micro devices in contact with or proximity contact withthe receiver substrate; and moving the donor substrate and the receiversubstrate apart leaving the selected micro devices on the receiversubstrate.

Some embodiments further comprise weakening the donor force bonding themicro devices to the donor substrate to assist micro device transfer.

In some embodiments, the donor force for the selected micro devices isweakened to improve selectivity in micro device transfer. In someembodiments, the receiver force is generated selectively to improveselectivity in micro device transfer.

Some embodiments further comprise weakening the donor force using laserlift off.

Some embodiments further comprise modulating the force by magneticfield.

Some embodiments further comprise weakening the donor force by heatingan area of the donor substrate.

Some embodiments further comprise modulating the receiver force byheating the receiver substrate.

In some embodiments the heating is performed by passing a currentthrough the contact pads. In some embodiments the receiver force isgenerated by mechanical grip.

Some embodiments further comprise performing an operation on thereceiver substrate so that the contact pads permanently bond with theselected micro devices.

In some embodiments the receiver force is generated by electrostaticattraction between the selected micro devices and the receiversubstrate. In some embodiments the receiver force is generated by anadhesive layer positioned between the selected micro devices and thereceiver substrate.

Some embodiments further comprise removing the donor force; and applyinga push force to selected micro devices to move the devices toward thereceiver substrate.

In some embodiments the push force is created by a sacrificial layerdeposited between the selected micro device and the donor substrate.

According to another aspect there is provided a receiver substratestructure comprising: an array of landing areas for holding microdevices from a donor substrate selectively, each landing areacomprising: at least one contact pad for coupling or connecting a microdevice to at least one circuit or a potential in the receiver substrate;and at least one force modulation element for creating a receiver forcefor holding micro devices on the receiver substrate. For clarity, thearea where the micro device sits on the receiver substrate is called thelanding area.

In some embodiments the force modulation element is an electrostaticstructure. In some embodiments the force modulation element is amechanical grip. In some embodiments, for each landing area, a sameelement acts as the force modulation element and the contact pad.

The foregoing and additional aspects and embodiments of the presentdisclosure will be apparent to those of ordinary skill in the art inview of the detailed description of various embodiments and/or aspects,which is made with reference to the drawings, a brief description ofwhich is provided next.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the disclosure will becomeapparent upon reading the following detailed description and uponreference to the drawings.

FIG. 1A shows a donor substrate and a receiver substrate before thetransfer process begins.

FIG. 1B shows a donor substrate and a receiver substrate before thetransfer process begins.

FIG. 2A shows a flowchart of modulating at least one of the donor orreceiver forces after donor and receiver substrates are in contact orproximity with each other.

FIG. 2B shows a flowchart of modulating the donor forces in advance andmodulating receiver forces if needed after donor and receiver substratesare in contact or proximity with each other.

FIG. 2C shows flowchart of modulating the receiver forces in advance andmodulating donor forces if needed after donor and receiver substratesare in contact or proximity with each other.

FIG. 3A shows the step of aligning the donor and receiver substrates

FIG. 3B shows the step of moving the substrates together within adefined distance margin.

FIG. 3C-1 shows one embodiment of modulating the forces by applyingreceiver forces selectively.

FIG. 3C-2 shows one embodiment of modulating the forces by weakening thedonor force selectively and applying receiver force globally.

FIG. 3D shows one embodiment of modulating the forces by applyingreceiver and weakening donor forces selectively.

FIG. 3E shows the step of moving the substrate apart.

FIG. 4A shows a donor substrate with different micro devices interleavedand the corresponding contact pads in the receiver substrate are alignedwith each micro device accordingly enabling transferring different microdevices at once.

FIG. 4B shows a donor substrate with different micro devices in groupsand the corresponding contact pads in the receiver substrate are alignedwith each micro device accordingly enabling transferring different microdevices at once.-4C show arrangements with different pitches of microdevices and contact pads.

FIG. 4C shows a donor substrate with different micro devices interleavedand only one set of the corresponding contact pads in the receiversubstrate with one of the micro device types is aligned with each microdevice accordingly so multiple transferring process is needed totransfer all different types of micro devices.

FIG. 5A shows selective and global heating elements incorporated intosubstrates.

FIG. 5B shows one embodiment for pattering selective and global heatingelements incorporated into substrates.

FIG. 5C shows use of external sources to selectively heat up at leastone substrate.

FIG. 6A shows a flowchart of method 1100 for selectively transferringmicro devices from a donor substrate to a receiver substrate.

FIG. 6B shows the step of preparing the donor and receiver substratesfor selective transfer.

FIG. 6C shows the step of aligning the substrates.

FIG. 6D shows the step of moving the substrates toward each other withina predefined distance margin.

FIG. 6E shows the step of creating receiver forces by curing theadhesive (e.g. applying pressure or heat). This can be globally orselectively.

FIG. 6F shows the step of reducing donor forces if needed. This can beglobally or selectively.

FIG. 6G shows the step of moving the substrates away from each other.

FIG. 7A shows other possible arrangements of adhesive on receiversubstrate.

FIG. 7B shows a contact pad with a cut out before and after applicationof an adhesive.

FIG. 8 shows a stamping process that can be used to apply adhesive tocontact pads.

FIG. 9 shows a flowchart of method 1200 for selectively transferringmicro devices from a donor substrate to a receiver substrate.

FIG. 10 shows a donor substrate and a receiver substrate setup toperform method 1200.

FIG. 11A shows the step of aligning donor and receiver substrates.

FIG. 11B shows the step of moving donor and receiver substrates to adefined distance margin while mechanical force is loose.

FIG. 11C shows the step of increasing mechanical forces.

FIG. 11D shows the step of reducing donor forces if needed (this stepcan be done in advance as well).

FIG. 11E shows moving the donor and receiver substrates away from eachother.

FIG. 12A shows a flowchart of method 1300 for selectively transferringmicro devices from a donor substrate to a receiver substrate.

FIG. 12B shows a donor substrate and a receiver substrate setup toperform method 1300.

FIG. 13A shows the step of aligning the donor and receiver substrates.

FIG. 13B shows the step of moving the substrates within a predefineddistance margin.

FIG. 13C shows the step of creating receiver force by applying potentialto electrostatic elements. This can be done selectively or globally.

FIG. 13D shows the step of reducing the donor force if needed. This canbe done globally or selectively.

FIG. 13E shows the step of moving the substrates away.

FIG. 14A shows another alternative placement for electrostatic layer.

FIG. 14B shows another alternative placement for electrostatic layer.

FIG. 14C shows another alternative placement for electrostatic layer.

FIG. 14D shows another alternative placement for electrostatic layer.

FIG. 15A shows another alternative geometrie for micro devices andcontact pads.

FIG. 15B shows another alternative geometrie for micro devices andcontact pads.

FIG. 15C shows another alternative geometrie for micro devices andcontact pads.

FIG. 15D shows another alternative geometrie for micro devices andcontact pads.

FIG. 15E shows another alternative geometrie for micro devices andcontact pads.

FIG. 16 shows a flowchart of method 1400 for selectively transferringmicro devices from a donor substrate to a receiver substrate.

FIG. 17A shows the step of aligning the donor and receiver substrates.

FIG. 17B shows the step of moving the substrates to a predefineddistance margin from each other.

FIG. 17C shows one embodiment for the step of creating a receiver forceif needed. This can be globally or selectively. The force can be createdwith different method.

FIG. 17D shows applying a push force to the micro devices from the donorsubstrate. The push force from donor substrate should be selective.

FIG. 17E shows the step of moving substrate away.

FIG. 18A shows a platform for testing by biasing at least one of thedonor substrate or the receiver substrate to enable testing the microdevices for defects and performance. Here, the output of the microdevice is through the receiver substrate.

FIG. 18B shows a platform for testing by biasing at least one of thedonor substrate or the receiver substrate to enable testing the microdevices for defects and performance. Here, the output of the microdevice is through the donor substrate.

FIG. 19 shows a simplified biasing condition of receiver substrate fortesting the micro devices for defect and performance analysis.

While the present disclosure is susceptible to various modifications andalternative forms, specific embodiments or implementations have beenshown by way of example in the drawings and will be described in detailherein. It should be understood, however, that the disclosure is notintended to be limited to the particular forms disclosed. Rather, thedisclosure is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of an invention as defined by theappended claims.

DETAILED DESCRIPTION

Many micro devices, including light emitting diodes (LEDs), OrganicLEDs, sensors, solid state devices, integrated circuits, MEMS(micro-electro-mechanical systems) and other electronic components, aretypically fabricated in batches, often on planar substrates. To form anoperational system, micro devices from at least one donor substrate needto be selectively transferred to a receiver substrate.

Substrate and Transfer Structure

FIG. 1 shows a donor substrate 100 and receiver substrate 200, beforethe transfer process begins. Micro devices 102 a, 102 b, 102 c begin inan array attached to donor substrate 100. The receiver substrateconsists of an array of landing areas 202 a, 202 b, 202 c where themicro devices will sit. The landing areas 202 a, 202 b, 202 c eachinclude at least one force modulation element 204 a, 204 b, 204 c and atleast a contact pad 206 a, 206 b, 206 c. The force modulation elementand contact pads can be different as shown in FIG. 1A or can be the samestructure as shown in FIG. 1B. The micro devices 102 may be coupled orconnected to a circuit or a potential on the receiver substrate 200through contact pads 206 a, 206 b, 206 c. The force modulation elements204 a, 204 b, 204 c create a transfer force to hold the micro device 102a, 102 b, 102 c selectively on the receiver substrate 200 and separatethem from the donor substrate 100. The donor substrate 100 is thesubstrate upon which micro devices 102 are manufactured or grown oranother temporary substrate onto which they have been transferred. Microdevices 102 can be any micro device that is typically manufactured inplanar batches including LEDs, OLEDs, sensors, solid state devices,integrated circuit, MEMS, and other electronic components. Donorsubstrate 100 is chosen according to the manufacturing process for aparticular type of micro device 102. For example, in the case ofconventional GaN LEDs, donor substrate 100 is typically sapphire.Generally, when growing GaN LEDs, the atomic distance of donor substrate100 should match that of the material being grown in order to avoiddefects in the film. Each micro device 102 is attached to donorsubstrate 100 by a force, FD, determined by the manufacturing processand the nature of the micro devices 102. FD will be substantially thesame for each micro device 102. Receiver substrate 200 can be any moredesirable location for micro devices 102. It can be, for example, aprinted circuit board (PCB), a thin film transistor backplane, anintegrated circuit substrate, or, in the case of optical micro devices102 such as LEDs, a component of a display, for example a drivingcircuitry backplane. The landing area on the receiver substrate as shownin FIG. 1B refers to the location where micro device sits on thereceiver substrate and may consist of at least one contact pad 101 a andat least one force modulation element 101 b. Although in some of thefigures the landing area may be the same size as the contact pads 202,the contact pads 202 can be smaller than the landing area. Contact pads202 are the locations where micro devices may be coupled or directlyconnected to the receiver substrate 200. In this description, landingarea and contact pads are used interchangeably.

The goal in selective transfer is to transfer some, selected microdevices 102, from donor substrate 100 to receiver substrate 200. Forexample, the transfer of micro devices 102 a and 102 b onto contact pads206 a and 206 b without transferring micro device 102 c will bedescribed.

Transfer Process

Following steps describe a method of transferring selected micro devicesin an array of micro devices each of which is bonded to a donorsubstrate with a donor force to contact pads in an array on a receiversubstrate:

-   -   a. aligning the donor substrate and the receiver substrate so        that each of the selected micro devices are in line with a        contact pad on the receiver substrate;    -   b. moving the donor substrate and the receiver substrate        together until each of the selected micro devices are in contact        with or proximity with at least one contact pad on the receiver        substrate;    -   c. generating a receiver force that acts to hold the selected        micro devices to their contact pads;    -   d. moving the donor substrate and the receiver substrate apart        leaving the selected micro devices on the receiver substrate        while other non-selected micro devices from donor substrate        stays on donor substrate despite possible contact with or        proximity contact with the system substrate during steps b and        c.

If the donor force is too strong for receiver force to overcome fortransferring the micro device to the receiver substrate, the donor forcefor micro devices is weakened to assist micro device transfer. Inaddition, if the receiver force is applied globally or selectivereceiver force is not enough to transfer the micro devices selectively,the donor force for the selected micro devices is weakened selectivelyto improve selectivity in micro device transfer.

FIGS. 2A-2C show exemplary flowcharts of selective transfer methods1000A-1000C. FIG. 1 shows a donor substrate 100 and a receiver substrate200 suitable for performing any of methods 1000. Method 1000A will bedescribed with reference to FIGS. 3A-3E. Methods 1000B and 1000C areanalogous variations of method 1000A. One can use the combination ofmethods 1000A-1000C to further enhance the transfer process.

At 1002A donor substrate 100 and receiver substrate 200 are aligned sothat selected micro devices 102 a, 102 b are in line with correspondingcontact pads 202 a, 202 b, as shown in FIG. 3A. Micro device 102 c isnot to be transferred so, although shown as aligned, it may or may notalign with contact pad 202 c.

At 1004A, donor substrate 100 and receiver substrate 200 are movedtogether until the selected micro devices 102 a, 102 b are positionedwithin a defined distance of contact pads 202 a, 202 b, as shown in FIG.3B. The defined distance may correspond to full or partial contact butis not limited thereto. In other words, it may not be strictly necessarythat selected micro devices 102 a, 102 b actually touch correspondingcontact pads 202 a, 202 b, but must be near enough so that the forcesdescribed below can be manipulated.

At 1006A, forces between selected micro devices 102, donor substrate 100and receiver substrate 200 (and contact pads 202) are modulated so as tocreate a net force towards receiver substrate 200 for selected microdevices and a net force towards donor substrate 100 (or zero net force)for other micro devices 102 c.

Consider the forces acting one of the selected micro devices 102. Thereis a pre-existing force holding it to donor substrate 100, FD. There isalso a force generated between micro device 102 and receiver substrate200, FR, acting to pull or hold micro device 102 towards receiversubstrate 200 and cause a transfer. For any given micro device 102, whenthe substrates are moved apart, if FR exceeds FD the micro device 102will go with receiver substrate 200, while if FD exceeds FR the microdevice 102 will stay with donor substrate 100. There are several ways togenerate FR that will be described in later sections. However, once FRhas been generated, there are at least four ( 4 ) possible ways tomodulate FR and FD to achieve transfer of selected micro devices.

1. Weaken FD to be less than FR on micro devices selected for transfer

2. Strengthen FR to be greater than FD on micro devices selected fortransfer.

3. Weaken FR to be less than FD on micro devices NOT selected fortransfer

4. Strengthen FD to be greater than FR on micro devices NOT selected fortransfer

Different combinations and arrangements of the above are also possible.Using combinations may, in some cases, be desirable. For example, if therequired change in FD or FR is very high, one can use a combination ofmodulation of FD and FR to achieve the desired net forces for theselected and the non-selected micro devices. Preferably, FR can begenerated selectively and therefore act only on selected micro devices102 a, 102 b, as shown in FIG. 3C-1. FR can also be generated globallyand apply across all of receiver substrate 200 and therefore act onmicro devices 102 a, 102 b, 102 c, as shown in FIG. 3C-2 here donorforces may selectively get weakened. The landing area on the receiversubstrate may include a force modulation element to cause FR forcemodulation, fully or partially. Methods for selective and globalgeneration of FR will be described below, including adhesive, mechanicaland electrostatic and magnetic techniques. Additionally, examples offorce modulation elements in landing area are described below. However,one of skill in the art knows that different variations of the forcemodulation elements that are not listed here are possible. Moreover, itshould be understood that the shapes and structures of the contact padsand the force modulation elements are used for explanation and are notlimited to the ones used in this description.

In one embodiment, donor force FD is selectively weakened for selectedmicro devices 102 a, 102 b, so that FD′ is less than FR, as shown inFIG. 3D. This may be done, for example, using laser lift off techniques,lapping or wet/dry etching. In some cases, it may be desirable to useselective and global generation of FR simultaneously. For example, itmay be infeasible to generate a selective FR of sufficient magnitude toovercome FD′ alone. In that case, the global component of FR shouldpreferably remain small, ideally less than FD′, while the sum of theglobal and the selective components of FR is greater than FD′, but lessthan FD.

It should also be noted that activities performed during steps1002A-1006A can sometimes be interspersed with one another. For example,selective or global weakening of FD could take place before thesubstrates are brought together.

At 1008A, donor substrate 100 and receiver substrate 200 are movedapart, leaving selected micro devices 102 a, 102 b attached tocorresponding contact pads 202 a, 202 b, as shown in FIG. 3E. Once donorsubstrate 100 is separated from receiver substrate 200, furtherprocessing steps can be taken. For example, donor substrate 100 andreceiver substrate 200 can be re-aligned and steps 1002A to 1008A can berepeated in order to transfer a different set of micro devices 102 to adifferent set of contact pads 202. Additional layers can also bedeposited on top of or in between micro devices 102, for example, duringthe manufacture of a LED display, transparent electrode layers, fillers,planarization layers and other optical layers can be deposited.

FIG. 2B shows method 1000B; an alternative embodiment of method 1000A.

At 1002B, the force between micro devices 102 a, 102 b and donorsubstrate 100 are modulated globally (for all devices in an area ofdonor substrate) or selectively (for selected micro devices 102 a, 102 bonly) so as to weaken donor force, FD.

At 1004B donor substrate 100 and receiver substrate 200 are aligned sothat selected micro devices 102 a, 102 b are in line with correspondingcontact pads 202 a, 202 b.

At 1006B, donor substrate 100 and receiver substrate 200 are movedtogether until the selected micro devices 102 a, 102 b touch contactpads 202 a, 202 b. It may not be strictly necessary that selected microdevices 102 a, 102 b actually touch corresponding contact pads 202 a,202 b, but must be near enough so that the forces described below can bemanipulated.

At 1008B, if needed the forces between selected micro devices 102 andreceiver substrate 200 (and contact pads 202) are modulated so as tocreate a net force towards receiver substrate 200 for selected microdevices and a net force towards donor substrate 100 (or zero net force)for other micro devices 102 c.

At 1010B, donor substrate 100 and receiver substrate 200 are movedapart, leaving selected micro devices 102 a, 102 b attached tocorresponding contact pads 202 a, 202 b.

At 1012B, optional post processing is applied to selected micro devices102 a, 102 b. Once donor substrate 100 is separated from receiversubstrate 200, further processing steps can be taken. Additional layerscan be deposited on top of or in between micro devices 102, for example,during the manufacture of a LED display, transparent electrode layers,fillers, planarization layers and other optical layers can be deposited.Step 1012B is optional and may be applied at the conclusion of method1000A or 1000C as well.

FIG. 2C shows method 1000C; an alternative embodiment of method 1000A.

At 1002C, contact pads 202 a, 202 b corresponding to selected microdevices 102 a, 102 b are treated to create extra force upon contact. Forexample, an adhesive layer may be applied, as described in greaterdetail below.

At 1004C donor substrate 100 and receiver substrate 200 are aligned sothat selected micro devices 102 a, 102 b are in line with correspondingcontact pads 202 a, 202 b.

At 1006C, donor substrate 100 and receiver substrate 200 are movedtogether until the selected micro devices 102 a, 102 b touch contactpads 202 a, 202 b.

At 1008C, if needed the forces between selected micro devices 102 anddonor substrate 100 are modulated so as to create a net force towardsreceiver substrate 200 for selected micro devices and a net forcetowards donor substrate 100 (or zero net force) for other micro devices102 c.

At 1010B, donor substrate 100 and receiver substrate 200 are movedapart, leaving selected micro devices 102 a, 102 b attached tocorresponding contact pads 202 a, 202 b.

Multiple Applications

Any of the methods 1000A, 1000B, 1000C can be applied multiple times tothe same receiver substrate 200, using different or the same donorsubstrates 100 or the same donor substrate 100 using different receiversubstrates 200. For example, consider the case of assembling a displayfrom LEDs. Each pixel may comprise red, green and blue LEDs in acluster. However, manufacturing LEDs is more easily done in batches of asingle colour and on substrates that are not always suitable forincorporation into a display. Accordingly, the LEDs must be removed fromthe donor 100 substrate, possibly where they are grown, and placed on areceiver substrate, which may be the backplane of a display, in RGBclusters. In case, the color This is simplest when the pitch of thearray of pixels can be set to match the pitch of the array of LEDs onthe donor substrate.

When this is not possible, the pitches of each array can be setproportionally. FIGS. 4A and 4B show arrangements where the pitch of theLEDs on the donor substrate is one seventh the pitch of the contact padson the receiver substrate.

In general, however, matching the pitch of an array of pixels to thedonor substrate is likely to be infeasible. For example, one generallytries to manufacture LEDs with the smallest possible pitch on the donorsubstrate to maximize yield, but the pitch of the pixels and the arrayof contact pads on the receiver substrate is designed based on desiredproduct specifications such as size and resolution of a display. In thiscase, one may not be able to transfer all the LEDs in one step andrepetition of any of the methods 1000A, 1000B, 1000C will be necessary.Accordingly, it may be possible to design the donor substrate and thereceiver substrate contact pad array so that a portion of each pixel canbe populated during each repetition of any of methods 1000A, 1000B,1000C as shown in FIG. 4C At I, receiver substrate and donor substrateare not aligned. At II, all red LEDs are transferred. At III, all greenLEDs are transferred. At IV, all blue LEDs are transferred.Repositioning of donor substrate and receivers substrate is requiredbetween each transfer step.

Those of skill in the art will now understand that that additionalvariations and combinations of methods 1000A, 1000B and 1000C are alsopossible. Specific techniques and considerations are described belowthat will apply to any of methods 1000, alone or in combination.

Use of Heat for Force Modulation

Selective and global heating can be used in multiple ways to assist inmethod 1000A. For example, heat can be used in step 1008A to weaken FDor after step 1008A to create a permanent bond between micro devices 102and contact pads 202. In one embodiment, heat can be generated usingresistive elements incorporated into donor substrate 100 and/or receiversubstrate 200.

FIG. 5A shows selective and global heating elements incorporated intosubstrates. Selective heating elements 300 and global heating element302 may be incorporated into donor substrate 100 while selective heatingelements 304 and global heating element 306 may be incorporated intoreceiver substrate 200. In another embodiment, selective heating can beachieved using a patterned global heater, shown in plan view in FIG. 5B.

FD can be weakened by applying heat to the interface between a microdevice 102 and donor substrate 100. Preferably, selective heatingelements 300 are sufficient to heat the interface past a thresholdtemperature where micro devices 102 will detach. However, when this isnot feasible, global heater 302 can be used to raise the temperature toa point below the threshold while selective heaters 300 raise thetemperature further, only for selected micro devices 102 a, 102 b abovethe threshold. An environmental heat source, e.g. a hot room, cansubstitute for the global heater.

Heat can also be used to create a permanent bond between micro devices102 and contact pads 202. In this case, contact pads 202 should beconstructed of a material that will cure when heated, creating apermanent bond. Preferably, selective heating elements 304 aresufficient to heat contact pads 202 past a threshold temperature tocause curing. However, when this is not feasible, global heater 306 canbe used to raise the temperature to a point below the threshold forcuring while selective heaters 304 raise the temperature for selectedcontact pads 202 a, 202 b above the threshold. An environmental heatsource, e.g. a hot room, can substitute for the global heater. Pressuremay also be applied to aid in permanent bonding.

Other variations are possible. In some cases, it may be feasible formicro devices 102 or contact pads 202 to themselves act as the resistiveelements in selective heaters 300, 304. Heat can also be applied in aselective manner using lasers. In the case of lasers, it is likely thatat least one of the donor substrate 100 and the receiver substrate 200will have to be constructed of material that is at leastsemi-transparent to the laser being used. As shown in FIG. 5C, in onecase, shadow mask can be used to selectively block the laser from thenon-selected devices. Here, the shadow mask 501 is aligned with thereceiver substrate or donor substrate depending on direction of laser.Then laser can cover the either substrate partially or fully. In case ofpartial coverage, raster scan or step-and-repeat may be used to coverthe entire intended area on the substrate. To further improve the heattransfer from the laser, a layer with higher laser absorption rate canbe added to the force modulation element. It is possible to use thecontact pad as the force modulation element in the receiver substrate.

Adhesive Force Modulation

In another embodiment of selective transfer, FR is generated byadhesive. Here, the FR is modulated either by selective application ofadhesive to the landing area on the receiver substrate (or selectedmicro devices) or by selective curing of an adhesive layer. This methodcan be used in combination with weakening the donor force selectively orglobally and is compatible with any of the methods 1000A, 1000B, and1000C or any combination of them. Although, the following description isbased on 1000A similar approaches can be used for 1000B, 1000C and thecombination of the methods. In addition, the order of donor forceweakening step 1110 can be changed in reference to other steps withoutaffecting the results.

FIG. 6A shows a flowchart of method 1100, a modified version of method1000 specific to the use of adhesive to generate FR. FIG. 6B shows donorsubstrate 100 and receiver substrate 200 setup to perform method 1100.Donor substrate 100 is shown in cross section and receiver substrate 200is shown in cross section and plan view. Donor substrate 100 has anarray of micro devices 102 attached. Donor force FD acts to hold microdevices 102 to donor substrate 100.

Receiver substrate 200 has an array of contact pads 212 attached.Although FIG. 6B shows the force modulation element 500 connected to thecontact pads 212, they can be physically separated.

As shown in FIG. 6B, contact pads 212 a, 212 b are surrounded by a ringof adhesive 500. Adhesive 500 has been applied selectively to contactpads 212 where transfer of a micro device is desired so that when donorsubstrate 100 and receiver substrate 200 are moved together, microdevices 102 a, 102 b will make contact with adhesive 500 as well ascontact pads 212 a, 212 b.

Method 1100 will be explained with reference to FIGS. 6B-6F. At 1102,adhesive is selectively applied as shown in FIG. 6B.

At 1104 donor substrate 100 and receiver substrate 200 are aligned sothat selected micro devices 102 a, 102 b are in line with correspondingselected contact pads 212 a, 212 b, as shown in FIG. 6C.

At 1106, donor substrate 100 and receiver substrate 200 are movedtogether until selected micro devices 102 a, 102 b are in contact withcorresponding selected contact pads 212 a, 212 b and adhesive 500, asshown in FIG. 6D.

At 1108, receiver force, FR, is generated, as shown in FIG. 6E. FR isgenerated by adhesion between micro devices 102 a, 102 b, adhesive 500and at least one of contact pads 212 a, 212 b and receiver substrate200. FR acts to hold selected micro devices 102 to correspondingselected contact pads 212. Preferably, FR can be generated selectivelyby applying adhesive 500 selectively, as shown.

At 1110, donor force FD is selectively (or globally) weakened forselected micro devices 102 a, 102 b, so that FD′ is less than FR, asshown in FIG. 6F. The may be done, for example, using laser lift offtechniques, lapping or wet/dry etching. In another case, donor force FDcan be weakened for all the micro devices. In this case, forcemodulation is done by selective adhesive application to the selectedforce element on the receiver substrate. The order of FD and FRmodulation can be changed. This step may be eliminated if the adhesiveforce modulation is selective and FR is larger than FD.

At 1112, donor substrate 100 and receiver substrate 200 are moved apart,leaving selected micro devices 102 a, 102 b attached to correspondingselected contact pads 212 a, 212 b, as shown in FIG. 6G. Once donorsubstrate 100 is separated from receiver substrate 200, furtherprocessing steps can be taken. For example, donor substrate 100 andreceiver substrate 200 can be re-aligned and steps can be repeated inorder to transfer a different set of micro devices 102 to contact pads212. Additional layers can also be deposited on top of or in betweenmicro devices 102, for example, during the manufacture of a LED display,a transparent electrode layers, fillers, planarization layers and otheroptical layers can be deposited.

One possible additional step, at 1114, is curing adhesive 500. Curingmay create a permanent bond between micro devices 102 and contact pads212. In another embodiment, curing takes place as part of step 1108 andis part of generating FR. If several sets of selected micro devices 102are to be transferred to a common receiver substrate 200 curing may bedone after all the transfers are complete or after each set istransferred.

Adhesive 500 can be applied in many ways. For example, adhesive 500 canbe applied to any or all of micro devices 102, contact pads 212 orreceiver substrate 200. It will often be desirable that an electricalcoupling exist between a micro device 102 and its corresponding contactpad 202. In this case, the adhesive may be selected for itsconductivity. However, suitable conductive adhesives are not alwaysavailable. In any case, but especially when a conductive adhesive is notavailable, adhesives can be applied near contact pads or may cover onlya portion of the contact pad. FIG. 7A shows some other possiblearrangements of adhesive on receiver substrate 200, (I) including fourcorners, (II) opposite sides, (III) center and (IV) one side geometries.

In another embodiment, one or more cut-outs can be provided for theadhesive 500. FIG. 7B shows a contact pad 212 with a cut out (I) beforeand (II) after application of an adhesive.

The adhesive 500 can be stamped, printed or patterned onto the contactpads 212, micro devices 102 or receiver substrate 200 by any normallithography techniques. For example, FIG. 8 shows a stamping processthat can be used to apply adhesive 500 to, for example, contact pads212. Selectivity in generating FR can be achieved by selecting whichcontact pads 212 will receive adhesive 500. An analogous procedure canbe used to apply adhesive to micro devices 102 or receiver substrate200. At (I), a stamp with a profile matching the desired distribution ofadhesive 500 is wet. At (II), the stamp is brought into contact with thereceiver substrate 200 and selected micro devices 102. At (III),receiver substrate is now wet with adhesive and ready to receivetransfer of selected micro devices 102. Depending on the needs of theprocess, stamps with reverse profiles can also be used. In anotherembodiment, both the micro devices 102 and contact pads 212 may be wetwith adhesive.

Adhesive 500 may be selected so that it will cure when heat is applied.Any of the techniques described with regard to heating can be suitablyapplied by one of skill in the art, according to the needs of a specificapplication.

Mechanical Force Modulation

In another embodiment of selective transfer, FR is generated bymechanical force. Here, the FR is modulated by application of mechanicalforces between the landing area on the receiver substrate and the microdevice. This method can be used in combination with weakening the donorforce selectively or globally and is compatible with any of the methods1000A, 1000B, and 1000C or any combination of them. Although, thefollowing description is based on 1000A similar approaches can be usedfor 1000B, 1000C and the combination of the methods. In addition, theorder of donor force weakening step 1210 can be changed in reference toother steps without affecting the results.

In one example, differential thermal expansion or pressure force can beused to achieve a friction fit that will hold micro devices 102 tocontact pads 202.

FIG. 9 shows a flowchart of method 1200, a modified version of method1000A suitable for mechanical generation of FR. FIG. 10 shows a donorsubstrate 100 and a receiver substrate 200 setup to perform method 1200.Donor substrate 100 is shown in cross section and receiver substrate 200is shown in cross section and plan view. Donor substrate 100 has anarray of micro devices 102 attached. Donor force FD acts to hold microdevices 102 to donor substrate 100. Micro devices 102 and donorsubstrate 100 are shown as connected to ground 244.

Receiver substrate 200 has an array of contact pads 232 attached. In theembodiment shown, the array of contact pads 232 is of the same pitch asthe array of micro devices 102; i.e., there is one micro device 102 foreach contact pad 232. As discussed above, this need not be true,although it is preferable that the pitch of the array of contact pads232 and the pitch of the array of micro devices 102 be proportional asthis facilitates the transfer of multiple devices simultaneously.

Method 1200 will be described with reference to FIGS. 11A-11E. At 1202the substrates are prepared for mechanical force modulation. In case ofa mechanical grip, the grip is opened by different means. In one exampleheat is applied to force modulation element 222 which can be the same acontact pad on the landing area. Here, mechanical grip and contact padsare used interchangeably. However, it is obvious to one of skill in theart that the mechanical grip and contact pad can be different. It ispossible to integrate the mechanical grip in the micro devices as well.The heat can be applied globally or selectively using heaters 304causing the grip to open, as shown by the double arrows in FIG. 11A.Note that contact pads 222 are constructed with a central depression 224and peripheral walls 226. It should also be noted that a combination ofselective heaters 304 and global heater 306 or a combination ofselective heaters 304 and an environmental heat source or external heatsource in combination or alone could also be used.

At 1204, donor substrate 100 and receiver substrate are aligned so thatselected micro devices 102 a, 102 b are in line with correspondingcontact pads 222 a, 222 b, as shown in FIG. 11B.

At 1206, donor substrate 100 and receiver substrate 200 are movedtogether until the selected micro devices 102 a, 102 b fit into thespace defined by the peripheral walls of corresponding mechanical gripas shown in FIG. 11B. As noted above, each contact pad 222 isconstructed with a central depression 224 and peripheral walls 226.These features of contact pads 222 are sized so as to fit snugly arounda micro device 102. The material of the mechanical grips is chosen, inpart, due to thermal properties; specifically so that the mechanicalgrips have a higher coefficient of thermal expansion than micro devices102. Accordingly, when heat is applied to the mechanical grips theyexpand more than a micro device 102 would expand at the same temperatureso that the central depression and peripheral walls will be able toaccommodate a micro device 102 with a gap 228. The expanded size ofmechanical grip allows micro devices 102 to fit easily.

At 1208, a receiver force, FR, is generated. FR is generated byselectively cooling contact pads 222 corresponding to selected microdevices 102, causing peripheral walls 226 to contract around selectedmicro devices 102, closing gap 228 and exerting a compressive force onmicro device 102, holding it in place, as shown in FIG. 11C. Selectivitycan be achieved by selectively turning off selective heaters 304.

At 1210, donor force FD is selectively (or globally) weakened forselected micro devices 102 a, 102 b, so that FD′ is less than FR, asshown in FIG. 11D. This may be done, for example, using laser lift offtechniques, lapping or wet/dry etching. In some embodiments FD is weakerthan FR, in which case selective weakening of FD is not required. Thisstep may be eliminated if the mechanical force modulation is selectiveand the FR is larger than FD.

At 1212, donor substrate 100 and receiver substrate 200 are moved apart,leaving selected micro devices 102 a, 102 b attached to correspondingcontact pads 222 a, 222 b, as shown in FIG. 11E. Once donor substrate100 is separated from receiver substrate 200, further processing stepscan be taken. For example, donor substrate 100 and receiver substrate200 can be re-aligned and steps can be repeated in order to transfer adifferent set of micro devices 102 and to contact pads 222. Additionallayers can also be deposited on top of or in between micro devices 102,for example, during the manufacture of a LED display, transparentelectrode layers, fillers, planarization layers and other optical layerscan be deposited.

Electrostatic Force Modulation

In another embodiment of selective transfer, FR is generated by anelectrostatic force or magnetic force. In case of magnetic force acurrent passes through a conductive layer instead of charging aconductive layer for electrostatic force. Although the structures hereare used to describe the electrostatic force similar structures can beused for magnetic force. Here, the FR is modulated by application ofselective electrostatic forces between the landing area on the receiversubstrate and the micro device. This method can be used in combinationwith weakening the donor force selectively or globally and is compatiblewith any of the methods 1000A, 1000B, and 1000C or any combination ofthem. Although, the following description is based on 1000A similarapproaches can be used for 1000B, 1000C and the combination of themethods. In addition, the order of donor force weakening step 1410 canbe changed in reference to other steps without affecting the results.

In another embodiment of selective transfer, FR is generated by anelectrostatic force. Here, the FR is modulated by application ofselective electrostatic forces between the landing area on the receiversubstrate and the micro device. This method can be used in combinationwith weakening the donor force selectively or globally and is compatiblewith any of the methods 1000A, 1000B, and 1000C or any combination ofthem. Although, the following description is based on 1000A similarapproaches can be used for 1000B, 1000C and the combination of themethods. In addition, the order of donor force weakening step 1410 canbe changed in reference to other steps without affecting the results.

FIG. 12A shows a flowchart of method 1300, a modified version of method1000 suitable for electrostatic generation of FR. FIG. 12B shows a donorsubstrate 100 and a receiver substrate 200 setup to perform method 1300.Donor substrate 100 is shown in cross section and receiver substrate 200is shown in cross section and in plan view. Donor substrate 100 has anarray of micro devices 102 attached. Donor force FD acts to hold microdevices 102 to donor substrate 100. Micro devices 102 and donorsubstrate 100 are shown as connected to ground 244.

The landing area on the receiver substrate 200 has at least a contactpad 232 attached and a force modulation element 234.

Contact pads 232 are surrounded by a ring of conductor/dielectricbi-layer composite, hereinafter called an electrostatic layer 234. Theshape and location of force modulation element 234 can be changed in thelanding area and in relation to the contact pad. Electrostatic layer 234has a dielectric portion 236 and a conductive portion 238. Dielectricportion 236 comprises a material selected, in part, for its dielectricproperties, including dielectric constant, dielectric leakage andbreakdown voltage. The dielectric portion can also be part of the microdevice or a combination of the receiver substrate and the micro device.Suitable materials may include SiN, SiON, SiO, HfO and various polymers.Conductive portion 238 is selected, in part, for its conductiveproperties. There are many suitable single metals, bi-layers andtri-layers that can be suitable including Ag, Au and Ti/Au. Eachconductive portion 238 is coupled to a voltage source 240, via a switch242. Note that although conductive portions 238 are shown as connectedin parallel to a single voltage source 240 via simple switches 242, thisis to be understood as an illustrative example. Conductive portions 238might be connected to one voltage source 240 in parallel. Differentsubsets of conductive portions 238 may be connected to different voltagesources. Simple switches 242 can be replaced with more complexarrangements. The desired functionality is the ability to selectivelyconnect a voltage source 240, having a potential different than that ofthe micro devices 102, to selected conductive portions 238 when neededto cause an electrostatic attraction between the selected conductiveportions 238 and corresponding selected micro devices 102.

Method 1300 will be explained in conjunction with FIGS. 13A-13E. At1302, donor substrate 100 and receiver substrate are aligned so thatselected micro devices 102 a, 102 b are in line with correspondingcontact pads 232 a, 232 b, as shown in FIG. 13A.

At 1304, donor substrate 100 and receiver substrate 200 are movedtogether until the micro devices 102 come into contact with contact pads232, as shown is FIG. 13B.

At 1306, a receiver force, FR, is generated, as shown in FIG. 13C. FR isgenerated by closing switches 242 a, 242 b that connect conductiveportions 238 of electrostatic layers 234 to voltage source 240 creatingcharged conductive portions 238 at the potential of voltage source 240.Selected micro devices 102 a, 102 b, being at a different potential,e.g. ground potential (or other relative potential), will beelectrostatically attracted to conductive portions 238. Theelectrostatic charge can be generated by different potential levels. Forexample, for a 300 nm dielectric, to get a proper grip on a microdevice, a voltage difference between 20 V to 50 6V may need to beapplied to the electrostatic force element. However, this voltage can bemodified depending on the device, gap size, and the dielectric constant.

At 1308, donor force FD is selectively weakened for selected microdevices 102 a, 102 b, so that FD′ is less than FR, as shown in FIG. 13D.This may be done, for example, using laser lift off techniques, lappingor wet/dry etching.

At 1310, donor substrate 100 and receiver substrate 200 are moved apart,leaving selected micro devices 102 a, 102 b attached to correspondingcontact pads 232 a, 232 b, as shown in FIG. 13E. Once donor substrate100 is separated from receiver substrate 200, further processing stepscan be taken and the ground 244 may be removed. For example, donorsubstrate 100 and receiver substrate 200 can be re-aligned and steps canbe repeated in order to transfer a different set of micro devices 102and to contact pads 232. Additional layers can also be deposited on topof or in between micro devices 102, for example, during the manufactureof a LED display, transparent electrode layers, fillers, planarizationlayers and other optical layers can be deposited. It should be notedthat FR will cease to operate if the connection to voltage source 240 isremoved. Accordingly, further processing steps to create a permanentbond between micro devices 102 and contact pads 232 are desirable.Curing contact pads 232, as described above, is a suitable furtherprocessing step that will create such a bond and enable further workingor transporting receiver substrate 200.

In other embodiments, electrostatic layer 234 can take on otherconfigurations. FIG. 14 shows some alternative placements forelectrostatic layer 234. Possible alternative placements ofelectrostatic layer 234 relative to each contact pad 232 include: (A)four corners, (B) opposite sides, (C) center and (D) one side. Those ofskill in the art will now be able to design a configuration suitable toparticular applications.

In other embodiments, the geometry of contact pads 232, electrostaticlayer 234 and micro devices 102 can be changed to varying effect. FIG.15 illustrates some possible alternative geometries. FIG. 15A shows anembodiment where electrostatic layer 234 extends above the top ofcontact pad 232 to form a hollow 240 and micro device 102 has a mesa 242that will fit within hollow 240. FIG. 15B shows an embodiment whereelectrostatic layer 234 extends above the top of contact pad 232 to forma hollow 240 and micro device 102 has an extension 244 attached to itthat will fit within hollow 240. Extension 240 may be made of the samematerial as contact pad 232 so that later curing will fuse extension 244and contact pad 232. Sloping geometries, as shown in FIG. 15E, are alsopossible. Geometries with mesa 242 or extension 244 can help guide microdevices 102 into contact pads 232 and insure a proper fit and preventtilting of micro devices 102 when detaching from donor substrate 100.Preferably, the geometry of micro devices 102 and contact pads 232 arechosen to match so as to maximize the electrostatic force.

FIG. 15C shows an embodiment where electrostatic layer 234 forms ahollow 240, but conductive portion 238 remains in the same plane ascontact pad 232. FIG. 15D shows an embodiment where electrostatic layer234 forms a hollow 240, but also overlaps with contact pad 232 andconductive portion 238 is in a different plane than contact pad 232,allowing the fine tuning of the electrostatic force.

Transfer of Micro Devices of Different Heights

In another embodiment of selective transfer, the force on the donorsubstrate is modulated to push the device toward the receiver substrate.In one example, after removing the donor force other forces such aselectrostatic forces can be used to push the device toward the receiversubstrate. In another case, a sacrificial layer can be used to create apush force in presence of heat or light sources. To selectively createthe push force, a shadow mask can be used for applying a light source(e.g. laser) to the selected micro devices. In addition, the FR can begenerated by one of aforementioned methods (e.g., mechanical, heating,adhesive, electrostatic). For example, the FR can be modulated byapplication of selective electrostatic forces between landing area onthe receiver substrate and the micro device. This method is compatiblewith any of the methods 1000A, 1000B, and 1000C or any combination ofthem. Although, the following description is based on 1000A, similarapproaches can be used for 1000B, 1000C and the combination of themethods. In addition, the order of donor force modulation step 1410 canbe changed in reference to other steps without affecting the results.However, the most reliable results can be achieved by applying the FRfirst and then applying the push force to the micro device.

FIG. 16 shows a flowchart of method 1400 based on electrostatic FR.However, other FR forces can be applied as well. Method 1400 is amodified version of method 1300 and is particularly suited tosimultaneous transfer of micro devices 102 of different heights. At1402, donor substrate 100 and receiver substrate are aligned so thatselected micro devices 102 a, 102 b are in line with correspondingcontact pads 232 a, 232 b, as shown in FIG. 17A. Note that micro device102 a is of a different height than micro device 102 b.

At 1404, donor substrate 100 and receiver substrate 200 are movedtogether until the micro devices 102 are close enough for electrostaticFR to act on micro devices 102. Donor substrate 100 and receiversubstrate 200 may be held so that no micro devices 102 make contact withcontact pads 232 or, as shown in FIG. 17B, substrates 100, 200 may stopapproaching when some micro devices 102 make contact with contact pads232.

At 1406, a receiver force, FR, is generated, as shown in FIG. 17C. FR isgenerated by closing switches 242 a, 242 b that connect conductiveportions 238 of electrostatic layers 234 to voltage source 240 creatingcharged conductive portions 238 at the potential of voltage source 240.Selected micro devices 102 a, 102 b, being at a different potential,e.g. ground potential, will be electrostatically attracted to conductiveportions 238.

At 1408, donor force FD is selectively weakened for selected microdevices 102 a, 102 b, so that FD′ is less than FR. This may be done, forexample, using laser lift off techniques, lapping or wet/dry etching. Atthis point, micro devices 102 a, 102 b will detach from donor substrate100. Micro device 102 b will jump the gap to their corresponding contactpads 232 a, 232 b on receiver substrate 200.

At 1410, donor substrate 100 and receiver substrate 200 are moved apart,leaving selected micro devices 102 a, 102 b attached to correspondingcontact pads 232 a, 232 b, as shown in FIG. 17E. Once donor substrate100 is separated from receiver substrate 200, further processing stepscan be taken. For example, donor substrate 100 and receiver substrate200 can be re-aligned and steps can be repeated in order to transfer adifferent set of micro devices 102 and to contact pads 232. Additionallayers can also be deposited on top of or in between micro devices 102,for example, during the manufacture of a LED display, transparentelectrode layers, fillers, planarization layers and other optical layerscan be deposited. It should be noted that FR will cease to operate ifthe connection to voltage source 240 is removed. Accordingly, furtherprocessing steps to create a permanent bond between micro devices 102and contact pads 232 are desirable. Curing contact pads 232, asdescribed above, is a suitable further processing step that will createsuch a bond and enable further working or transporting receiversubstrate 200.

One application of this method is development of displays based onmicro-LED devices. An LED display consists of RGB (or other pixelpatterning) pixels made of individual color LEDs (such as red, green orblue or any other color). The LEDs are manufactured separately and thentransferred to a backplane. The backplane circuit actively or passivelydrives these LEDs. In the Active form each sub-pixel is driven by atransistor circuit by either controlling the current, the ON time, orboth. In the Passive form, each sub-pixel can be addressed by selectingthe respective row and column and is driven by an external drivingforce.

The LEDs conventionally are manufactured in the form of single colorLEDs on a wafer and patterned to individual micro-devices by differentprocess such as etching. As the pitch of the LEDs on their substrate isdifferent from their pitch on a display, a method is required toselectively transfer them from their substrate to the backplane. TheLEDs' pitch on their substrate is the minimum possible to increase theLED manufacturing yield on a wafer, while the LED pitch on the backplaneis dictated by the display size and resolution. According to methodsimplemented here, one can modulate the force between the LED substrateand the micro-LEDs and uses any of the technique presented here toincrease the force between selected LED and backplane substrate. In onecase, the force for LED wafer is modulated first. In this case, theforce between LED devices and substrate is reduced either by laser,backplane etching, or other methods. The process can selectively weakenthe connection force between selected LEDs for transfer and the LEDsubstrate or it can be applied to all the devices to reduce theconnection force of all the LED devices to the LED substrate. In oneembodiment, this is accomplished by transferring all LEDs from theirnative substrate to a temporary substrate. Here, the temporary substrateis attached to the LEDs from the top side, and then the first substrateis removed either by polishing and/or etching or laser lift off. Theforce between the temporary substrate and the LED devices is weaker thanthe force that the system substrate can selectively apply to the LEDs.To achieve that a buffer layer may be deposited on the temporarysubstrate first. This buffer layer can be a polyamide layer. If thebuffer layer is not conductive, to enable testing the devices aftertransfer to the temporary and system substrate, an electrode before orafter the buffer layer will be deposited and patterned. If the electrodeis deposited before the buffer layer, the buffer layer maybe patternedto create an opening for contact.

In another method, the LED connection-force modulation happens after theLED substrate and the backplane substrate are in contact and the systemsubstrate forces to LED are selectively modulated by the aforementionedmethods presented here. The LED substrate force modulation can be doneprior to the backplane substrate force modulation as well.

As the force holding the LEDs to the backplane substrate after transferis temporary in most of the aforementioned methods, a post processingstep may be needed to increase the connection reliability to thebackplane substrate. In one embodiment, high temperature (and/orpressure can be used). Here, a flat surface is used to apply pressure tothe LEDs while the temperature is increased. The pressure increasesgradually to avoid cracking or dislocation of the LED devices. Inaddition, the selective force of the backplane substrate can stay activeduring this process to assist the bonding.

In one case, the two connections required for the LED are on thetransfer side and the LED is in full contact with the backplane afterthe transfer process. In another case, a top electrode will be depositedand patterned if needed. In one case, a polarization layer can be usedbefore depositing the electrode. For example a layer of polyamide can becoated on the backplane substrate. After the deposition, the layer canbe patterned to create an opening for connecting the top electrode layerto system substrate contacts. The contacts can be separated for each LEDor shared. In addition, optical enhancement layers can be deposited aswell before or after top electrode deposition.

Testing Process

Identifying defective micro devices and also characterizing the microdevices after being transferred is an essential part of developing ahigh yield system since it can enable the use of repair and compensationtechniques.

In one embodiment shown in FIG. 18, the receiver substrate is put intest mode during a transfer process. If needed, the donor substrate maybe biased for test mode. If the micro device is an optoelectronicdevice, a sensor 1810 (or sensor array) is used to extract the opticalcharacteristics of the transferred devices. Here, the receiver substrateis biased so that only the selected device 1802 is activated throughselected contact pads 1804. Also, unselected devices 1806 staydeactivated and unselected pads 1808 stay inactive to prevent anyinterference. For connectivity testing, the micro device is biased to beactive (for the LED case, it emits light). If a micro device is notactive, the device can be flagged as defective. In another test, themicro device is biased to be inactive (for the LED case, it does notemit light). If a micro device is active, the device can be flagged asdefective. FIG. 19 shows an example of a pixel biasing condition foractivating or deactivating a micro device. Here, the micro device 1906is coupled 1908 to a bias voltage 1910 (supply voltage) to becomeactivated. For deactivating the micro device 1906, it is disconnectedfrom the voltages. Here, the donor substrate 1900 can be biased forenabling the test. In another case, the micro devices are tested duringpost processing. While a surface is used to apply pressure to thedevices to create permanent bonding, the circuit is biased to activatethe micro devices. The surface can be conductive so that it can act asanother electrode of the micro devices (if needed). The pressure can beadjusted if a device is not active to improve any malfunction in theconnection to the receiver substrate. Similar testing can be performedto test for open defective devices. For performance testing, the microdevice is biased with different levels and its performance (for the LEDcase, its output light and color point) is measured.

In one case, the defective devices are replaced or fixed before applyingany post processing to permanently bond the device into receiversubstrate. Here, the defective devices can be removed before replacingit with a working device. In another embodiment, the landing area on thereceiver substrate corresponding to the micro devices comprises at leasta contact pad and at least a force modulation element.

It should be understood that various embodiments in accordance with andas variations of the above are contemplated.

In another embodiment, the net transfer forces are modulated byweakening the donor force using laser lift off. In another embodiment,the net transfer forces are modulated by weakening the donor force usingselectively heating the area of the donor substrate near each of theselected micro devices. In another embodiment, the net transfer forcesare modulated by selectively applying adhesive layer to the microdevices. In another embodiment, a molding device is used to apply theadhesive layer selectively. In another embodiment, printing is used toapply the adhesive layer selectively. In another embodiment, a postprocess is performed on the receiver substrate so that the contact padspermanently bond with the selected micro devices. In another embodiment,the post process comprises heating the receiver substrate. In anotherembodiment, the heating is done by passing a current through the contactpads. In another embodiment, the method is repeated using at least oneadditional set of selected micro devices and corresponding contact pads.In another embodiment, the contact pads are located inside anindentation in the receiver substrate and each selected micro devicefits into one such indentation. In another embodiment, the pitch of thearray of micro devices is the same as the pitch of the array of contactpads. In another embodiment, the pitch of the array of micro devices isproportional to the pitch of the array of contact pads. In anotherembodiment, each of the selected micro devices comprises a protrusionand the contact pads comprise a depression sized to match the protrusionon each micro device. In another embodiment, the net transfer forces aremodulated by generating electrostatic attraction between the selectedmicro devices and the receiver substrate. In another embodiment, theelectrostatic forces are applied to the entire array of micro devices onthe donor substrate by a force element on the receiver substrate orbehind the receiver substrate. In another embodiment, the electrostaticforces are generated selectively by the force modulation element of thelanding area. In another embodiment, the force modulation element of thelanding area on the receiver substrate comprises a conductive elementnear each contact pad, each conductive element capable of being linkedto a voltage source in order to sustain an electrostatic charge. Inanother embodiment, each conductive element comprises one or moresub-elements. In another embodiment, the sub-elements are distributedaround the contact pad. In another embodiment, each conductive elementsurrounds a contact pad. In another embodiment, the force modulationelement of the landing area on the receiver substrate comprises aconductive layer and a dielectric layer throughout a substantial portionof the landing area, the conductive layer capable of being linked to avoltage source in order to sustain an electrostatic charge. In anotherembodiment, the donor substrate and the receiver substrate are broughtclose together, but the selected micro devices and the contact pads donot touch until after the net transfer forces are modulated whereuponthe selected micro devices move across the small gap to the contactpads. In another embodiment, the height of the selected micro devicesdiffer. In another embodiment, the contact pads are concave. In anotherembodiment, the force modulation element of the receiver substrategenerates a mechanical clamping force. In another embodiment, themechanical force modulation element forms part of at least one contactpad. In another embodiment, the mechanical force modulation elements areseparate from the contact pad. In another embodiment, the mechanicalforce modulation is created by thermal expansion or compression of atleast one of the force modulation element or micro device. In anotherembodiment, each contact pad has a concave portion and each selectedmicro device is inserted into a concave portion of a contact pad.

In another embodiment, the receiver substrate is heated before the donorsubstrate and the receiver substrate are moved together so that theconcave portion of the contact pads expands to be larger than a selectedmicro device and the receiver substrate is cooled before the donorsubstrate and the receiver substrate are moved apart so that the concaveportion of the contact pads contracts around the selected micro devicesand provides the receiver force via mechanical clamping of the selectedmicro devices.

In another embodiment, the force modulation element in the landing areaof the receiver substrate is an adhesive layer positioned between theselected micro devices and the receiver substrate. In anotherembodiment, the adhesive layer is conductive. In another embodiment, aportion of each of the contact pads on the receiver substrate is coatedwith an adhesive layer. In another embodiment, a portion of each of theselected micro devices is coated with an adhesive layer. In anotherembodiment, a portion of the area near the contact pads is coated withan adhesive layer.

In another embodiment, the net transfer force is modulated both on thedonor substrate with at least one of the aforementioned methods and onthe receiver substrate with at least one of the described methods.

While particular implementations and applications of the presentdisclosure have been illustrated and described, it is to be understoodthat the present disclosure is not limited to the precise constructionand compositions disclosed herein and that various modifications,changes, and variations can be apparent from the foregoing descriptionswithout departing from the spirit and scope of an invention as definedin the appended claims.

What is claimed is:
 1. A method of transferring selected micro devicesin an array of micro devices each of which is bonded to a donorsubstrate with a donor force to contact pads in an array on a receiversubstrate, the method comprising: aligning the donor substrate and thereceiver substrate so that each of the selected micro devices is in linewith a contact pad on the receiver substrate; moving the donor substrateand the receiver substrate together until each of the selected microdevices is in contact or proximity with a respective contact pad on thereceiver substrate; generating a receiver force that acts to hold theselected micro devices to their contact pads; and moving the donorsubstrate and the receiver substrate apart leaving the selected microdevices on the receiver substrate.
 2. The method of claim 1 furthercomprising weakening the donor force bonding the micro devices to thedonor substrate to assist micro device transfer.
 3. The method of claim2 wherein the donor force for the selected micro devices is weakened toimprove selectivity in micro device transfer.
 4. The method of claim 1wherein the receiver force is generated selectively to improveselectivity in micro device transfer.
 5. The method of claim 1 furthercomprising weakening the donor force using laser lift off.
 6. The methodof claim 1 further comprising weakening the donor force by heating anarea of the donor substrate.
 7. The method of claim 4 further comprisingmodulating the receiver force by heating the receiver substrate.
 8. Themethod of claim 7 wherein the heating is performed by passing a currentthrough the contact pads.
 9. The method of claim 4 wherein the receiverforce is generated by mechanical grip.
 10. The method of claim 1 furthercomprising performing an operation on the receiver substrate so that thecontact pads permanently bond with the selected micro devices.
 11. Themethod of claim 4 wherein the receiver force is generated byelectrostatic attraction between the selected micro devices and thereceiver substrate.
 12. The method of claim 1 wherein the receiver forceis generated by an adhesive layer positioned between the selected microdevices and the receiver substrate.
 13. The method of claim 1 furthercomprising: removing the donor force; and applying a push force toselected micro devices to move the devices toward the receiversubstrate.
 14. The method of claim 13 wherein the push force is createdby a sacrificial layer deposited between the selected micro device andthe donor substrate.
 15. A receiver substrate structure comprising: anarray of landing areas for holding micro devices from a donor substrateselectively, each landing area comprising: at least one contact pad forcoupling a micro device to a circuit or a potential in the receiversubstrate; and at least one force modulation element for creating areceiver force for holding a micro devices on the receiver substrate.16. The receiver substrate structure of claim 15 wherein the forcemodulation element is an electrostatic structure.
 17. The receiversubstrate structure of claim 15 wherein the force modulation element isa mechanical grip.
 18. The receiver substrate structure of claim 15wherein for each landing area, a same element acts as the forcemodulation element and the contact pad.